Optical devices employing an optical thresholder

ABSTRACT

An optical device for use with an optical input beam comprises and optical thresholding device having a predetermined threshold level, and is positioned along an optical path defined by the propagation direction of the optical input beam. A source generates a control beam through the optical thresholding device, wherein if the combined intensity of the optical input beam and the control beam is large enough to exceed the threshold level of the thresholding device, the optical beam passes through he thresholding device. The thresholding device attenuates the optical beam as it passes therethrough. In a preferred embodiment, the optical thresholding device is a saturable absorber. When the device is configured as an optical comparator, the intensity of the optical input beam is large enough to exceed the threshold level of the thresholding device, the thresholding device saturates and turns transparent so that the control beam passes through the thresholding device as an optical indicator beam and the optical input beam passes through the thresholding device. When configured as an optical signal attenuator and the intensity of the optical input signal is negligible compared to that of the control beam the combined intensity of the optical input signal and the control beam do not saturate the thresholding device.

CROSS REFERENCE

[0001] This application is related to U.S. patent application Ser. No.09/133,032, filed on Aug. 11, 1998, entitled “Saturable Absorber BasedOptical Inverter”; U.S. patent application Ser. No. 09/343,733, filed onJun. 30, 1999, entitled “Repetitive Absorptive Thresholding OpticalQuantizer”; U.S. patent application Ser. No. 09/345,295, filed on Jun.30, 1999, entitled “Delayed Pulse Saturable Absorber-BasedDownward-Folding Optical A/D”; and U.S. patent application Ser. No.09/133,038, filed on Aug. 11, 1998, entitled “Upward Folding SuccessiveApproximation Optical Analog-to-Digital Converter and Method forPerforming Conversion”.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to an optical device employingan optical thresholder and, more particularly, to an optical comparatorthat employs an optical thresholding device positioned in a propagationpath of an optical input beam that provides an indicator signalindicative of whether the intensity of the input beam is above athreshold.

[0004] 2. Discussion of the Related Art

[0005] Advances in signal processing technology, including the need forgreater processing speeds, increased channel bandwidths and improvedtransmission reliability, has resulted in a steadily growing focus onthe optical domain and the vast potential that lies therein with respectto these parameters. However, optical technology, as compared toelectrical and radio frequency based technology, lacks the necessarytechnical sophistication in many areas. Particularly, the desire forhigh-speed, large-bandwidth processing devices employing digital opticshas been hampered by the lack of many basic optical devices andtechnologies readily available in the RF domain.

[0006] Optical comparators comprise an example of such a device that hasnot heretofore met basic design requirements. Similarly, variableoptical signal attenuator is another example of a device that has notheretofore met basic attenuator design requirements. An opticalcomparator that compares an optical signal with an optical threshold andindicates whether the threshold has been exceeded and/or an opticalsignal attenuator that decreases the intensity of an optical signal by apredetermined amount with little or no reliance on RF technology wouldreduce system complexity and provide for high speed and large bandwidthprocessing advances.

[0007] What is needed is an efficient, cost effective and reliableoptical comparator and variable optical signal attenuator that hasapplication to be used in an optical signal processing system. It istherefore an object of the present invention to provide such an opticalcomparator and an optical signal attenuator.

SUMMARY OF THE INVENTION

[0008] In accordance with the teachings of the present invention, twooptical devices are disclosed that employ an optical thresholding devicepositioned in the propagation path of an optical input beam.

[0009] For the optical comparator the optical thresholding devicesaturates and turns transparent if the intensity of the optical beamthat impinges it is above a predetermined threshold level which isdesigned into the device. If the input beam to the optical comparatorsaturates the optical thresholder it outputs an indicator signalidentifying the saturation and that the threshold has been exceeded. Thepresence of an indicator signal gives an indication of the intensity ofthe input beam. In one particular embodiment of the optical comparator,the optical thresholder is a saturable absorber that receives an opticalcontrol beam. If the saturable absorber saturates and turns transparent,the control beam passes through the thresholding device as an opticalindicator beam. Photodetectors can be provided to convert the opticalindicator beam to an electrical representation if desired. In analternate embodiment, the optical thresholder is a semiconductorsaturable absorber that is configured in a reverse-biased p-i-n layout,where the input beam generates electron/hole charge carriers in thesaturable absorber that generate a current flow that can be measured asthe indicator signal. When the semiconductor saturable absorbersaturates, it turns transparent and the maximum number of chargecarriers are generated.

[0010] For the optical signal attenuator the threshold level is variedand established by a control signal to selectively control the signalattenuation without saturating the optical thresholding device. In thisembodiment, the intensity of the optical beam is less than that of thecontrol signal.

[0011] Additional objects, advantages, and features of the presentinvention will become apparent from the following description andappended claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic plan view of an optical comparator employingan optical threshold device, according to an embodiment of the presentinvention.

[0013]FIG. 2 is a schematic plan view of an optical comparator employinga semiconductor optical thresholder, according to another embodiment ofthe present invention.

[0014]FIG. 3 is a schematic plan view of an optical signal attenuatoremploying an optical thresholder according to another embodiment of thepresent invention.

[0015]FIG. 4 is a graph of the transmission versus normalized inputintensity for the optical thresholder illustrated in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The following discussion of the preferred embodiments directed tooptical devices employing an optical thresholder is merely exemplary innature and is in no way intended to limit the invention or itsapplications or uses.

[0017]FIG. 1 is a schematic block diagram of an optical comparator 10,according to an embodiment of the present invention. The comparator 10receives an analog optical input beam 12 having an intensity between aminimum value, such as zero, and some known maximum value, which isnormally normalized to a value of unity, from a suitable optical source14. In one embodiment, the input beam 12 is modulated with an analog RFsignal. The modulation of the beam 12 can be by any suitable signal,both analog and digital. The output of the comparator 10 is an indicatorbeam that gives a representation of whether the intensity of the opticalbeam 12 exceeds a predetermined threshold as will be discussed below.

[0018] The comparator 10 includes a saturable absorber 16, acting as anoptical thresholding device (OTD), that is positioned along thepropagation path of the optical beam 12. A saturable absorber 16 is aknown optical device that is generally opaque or lossy in a normalstate, but when an optical beam having an intensity above a thresholdlevel of the absorber impinges it, the absorber will saturate and becometransparent or “bleached”. Saturable absorbers of this type that exhibitthese properties have been well known in the art for their use inmode-lock lasers. A discussion of saturable absorbers can be found inU.S. Pat. No. 5,303,256.

[0019] The absorber 16 measures the intensity of the beam 12, and if itexceeds the predetermined threshold level designed into the absorber 16,the beam 12 will pass as an optical signal output 18. Therefore, if theoptical beam 12 has a large enough intensity when it impinges theabsorber 16, it exceeds the threshold and passes to the output 18. Ifthe beam 12 does not have a large enough intensity to saturate absorber16, then it is absorbed by the absorber 16 and does not pass through.Saturable absorbers are used by way of a non-limiting example, in thatany optical thresholding device having the properties described hereincan be used as would be appreciated by those skilled in the art. U.S.Pat. Nos. 5,337,180 and 4,934,782 disclose other types of opticalthresholding devices that may be applicable.

[0020] The comparator 10 further includes a control laser 20 thatgenerates a laser beam 22, also referred to as a control or indicatorbeam. The indicator beam 22 is directed towards the absorber 16.Alternately, an optical reflector could be used to direct the indicatorbeam 22 into the saturable absorber 16 if the control laser is notdirected toward the absorber 16. The indicator beam 22 impinges theabsorber 16 from a direction relative to the propagation direction ofthe beam 12 so that the beam 22 does not couple with the beam 12, andobscure the signal strength of the beam 12. In this example, theindicator beam 22 and the input beam 12 are oriented at substantially90° relative to each other at the absorber 16. Also, for efficientoperation, the wavelengths of the beam 12 and the laser beam 20 shouldbe the same or nearly the same, although that is not required for allapplications. The wavelengths of the input beam 12 and the indicatorbeams 22 may need to be nearly the same because a saturable absorber mayonly operate over a certain range of wavelengths.

[0021] If the intensity of the beam 12 is large enough to saturate theabsorber 16 and make it transparent, the indicator beam 22 will passthrough the absorber 16 unabsorbed. If, however, the input beam 12 doesnot saturate the absorber 16, then the indicator beam 22 is absorbed bythe absorber 16 and does not pass through. The intensity of theindicator beam 22 is not large enough to saturate the absorber 16 byitself, and in one embodiment, the intensity of the indicator beam 22 ismuch less than the intensity of the input beam 12 when it is greaterthan zero. So, for practical purposes, the intensity of the indicatorbeam 22 is negligible with respect to saturating the absorber 16. Itshould be recognized that if the intensity of the beam 12 is largeenough to saturate an absorber 16, a small amount of light in the beam12 is still absorbed by the absorber 16 by the saturation process.

[0022] When the indicator beam 22 passes through the absorber 16 itcauses the comparator 10 to represent that the input beam 12 exceeds thethreshold. Also note that the output of the comparator 10 is themodified indicator beam 22 after passing through the absorber 16, notthe output signal output 18.

[0023] By carefully designing the threshold level for the absorber 16,an accurate comparison of the intensity of the beam 12 can be obtained.The threshold level can effectively be lowered if additional opticalintensity is input to the device. With reference to FIG. 2, adding athreshold control beam 32 of intensity one-half (½) of the thresholdintensity effectively lowers the threshold by one-half (½). Thethreshold control beam 32 from a laser 34 should be introduced such thatit does not couple or interfere with either the passed input beam 12 orthe indicator beam 22.

[0024] The indicator beams 22 that passes through the absorber 16 can bedirected to other optical devices (not shown) for optical processing inaccordance with the particular application that is used in connectionwith the comparator 10. Alternately, the indicator beam 22 can bedirected to a photodetector 30 to convert the indicator beam 22 into acorresponding electrical signal for applications requiring electricalsignals.

[0025]FIG. 3 is a schematic block diagram of a comparator 36 that is analternate embodiment of the comparator 10. Instead of using the genericoptical threshold device 16, the comparator 36 specifically uses asemiconductor saturable absorber 38. Semiconductor saturable absorbersabsorb light energy and create electron/hole charge pairs from theincident photons. The semiconductor saturable absorber 38 is configuredas a reverse-biased p-i-n structure, so that the reverse bias willinduce a photocurrent in the absorber 38 by this process ofphoto-absorption. A negative voltage potential is applied to theabsorber 38 to provide the reverse bias. Once the absorber 38 generatesall of the electron/hole pairs that it is capable of, it saturates andbecomes transparent. When the absorber 38 saturates, it produces itsmaximum current flow in the reverse-bias case. Thus, the semiconductorsaturable absorber can be used to produce an electrical signalproportional to the intensity of an impinging beam that it absorbs,allowing an indicator current to be produced.

[0026] As with the comparator 10 discussed above, the absorber 38 ispositioned along the propagation path of an input beam 42 so that thebeam 42 impinges the absorber 38. If the intensity of the beam 42 isgreater than the saturation threshold level of the absorber 38, theabsorber 38 will turn transparent and pass the beam 42 with someattenuation of intensity.

[0027] When the beam 42 impinges the absorber 38, electron/hole chargecarriers are created and the reverse-bias applied to the absorber 38causes the current generated by the carriers to flow to a transistor 44connected to the n layer of the absorber 38. This creates a voltagepotential on the emitter of the transistor 44 that is a measure of thecurrent generated. A current or voltage thresholding circuit 46 isprovided that measures the current from the saturable absorber 38. Ifthe beam 42 does not impinge the absorber 38, no current is generated.The transistor 44 and the thresholding circuit 46 can be replaced by anysuitable current detecting device to provide a measure of the currentflow in the absorber 38, as would be appreciated by those skilled in theart. Therefore, the measure of the current flow acts in the same manneras the indicator beams 22, discussed above, to give an indication ofwhether the intensity of the beam 42 is greater than the threshold. Thethreshold level of the absorber 38 car be designed in the manner asdiscussed above to provide a desired indication of the intensity of thebeam 42. Also, as in the embodiment shown in FIG. 1, the threshold levelcan be effectively lowered by use of the laser 34 emitting the thresholdcontrol beam 32.

[0028] Referring now to FIG. 4, there is illustrated another embodimentaccording to the present invention. The optical signal attenuator 100shown in this embodiment uses a control or indicator beam 22 that issignificantly larger than the optical input signal 12 and serves tochange the transmission characteristic of the optical tresholding device16, but does not cause it to bleach. The changing transmissiveness ofthe OTD 16 causes it to correspondingly attenuate the optical inputsignal. Many of the elements of the optical signal attenuator 100 areidentical in construction to like elements in the optical comparator 10illustrated in FIG. 1 described above, and accordingly, there have beenapplied to each item of the attenuator 100 in FIG. 4 a reference numeralcorresponding to the reference numeral that was applied to the like itemof the optical comparator described above and shown in FIG. 1.

[0029] The fundamental difference between the optical devices of FIGS. 1and 4 is that the optical input signal 12 of FIG. 4 is relatively small,typically about 1% of the threshold level of the OTD 16. The controlsource 20 generates a control beam 22 having an intensity that variesand is approximately the level of the threshold of the OTD 16. Thecontrol source 20 could either be a laser or an optical beam produced bysome other optical system. Hence, the variation in the intensity of thecontrol beam 22 serves to control the transmission of the OTD 16, butdoes not cause it to saturate, as will be subsequently described.

[0030] Also, with reference to FIG. 5, a characteristic graph 50 showsthe transmission T of the saturable absorber 16 plotted as a function ofthe intensity of the control beam normalized to the saturation intensityof the absorber. Note that the transmission varies from 0.3<T<0.8 whenthe normalized input intensity is varied from approximately 3 to 7.Hence the intensity of the control beam 22 can be varied to create anOTD with the desired transmission. For example, consider the situationwhere the intensity of the control beam 22 is adjusted to produce T=0.5.Since the intensity of the optical input beam 12 is so small compared tothe intensity of the control beam 22, it is effectively negligible tothe saturation process. Thus the control beam 22 controls the amount ofsaturation of the OTD 16, and hence its transmission. Accordingly, theoptical input signal 12 is attenuated by 0.5 by the saturable absorber16. Should the intensity of the control beam 22 be adjusted such thatT=0.3, then the optical input signal would be attenuated or reduced toprovide 0.3 of its intensity at the output port 18. It is not necessarythat a linear relationship exist between the control beam intensity andthe saturable absorber attenuation. All that is required is a monotonicmapping.

[0031] For comparison purposes recognize that an ideal saturableabsorber has a graph 52 in which the transmission curve resembles a stepfunction that transitions from T=0 to T=1 at the threshold intensity. Itshould be recognized that this could not be employed in the presentinvention because there is not a range of intensities that could causean incremental change in transmission of a signal through the absorber.

[0032] It should be noted that the present invention is amenable tointegrating into optical devices on to a single substrate and couldreplace today's optical signal attenuators that are packaged in largeelectronic boxes.

[0033] For the comparators and attenuator discussed above, the specificfrequencies of the modulating signal of the input beams are notimportant, except that there is a finite response time associated withthe devices.

[0034] The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion, and from the accompanyingdrawings and claims, that various, changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. An optical device for use with an optical inputbeam, comprising: an optical thresholding device having a predeterminedthreshold level, and being positioned along an optical path defined bythe propagation direction of the optical input beam; means forgenerating a control beam through said optical thresholding device,wherein if the combined intensity of the optical input beam and thecontrol beam is large enough to exceed the threshold level of saidthresholding device, the optical beam passes through the thresholdingdevice, said thresholding device attenuating the optical beam as itpasses therethrough.
 2. The device according to claim 1 wherein thecontrol beam and the input beam impinge the optical thresholding devicesuch that they do not interfere with each other.
 3. The device accordingto claim 1 wherein the optical thresholding device is a saturableabsorber, said optical beam and said control beam saturating thesaturable absorber and making it transparent if the combined intensityof said beams is greater than the threshold level of the absorber. 4.The device according to claim 3 wherein the saturable absorber providesan optical indicator beam if the optical input beam saturates theabsorber.
 5. The device according to claim 1, wherein it comprises anoptical comparator.
 6. The device according to claim 1 wherein theintensity of said optical beam and said control beam are below thethreshold level, the intensity of said control beam serving to controlthe transmission of said saturable absorber and hence the attenuation ofthe intensity of said optical beam.
 7. The device according to claim 6wherein it comprises an optical signal attenuator.
 8. An opticalcomparator for comparing an optical input beam with a threshold level,comprising: an optical source for generating a control beam; and anoptical thresholding device having a predetermined threshold level, saidoptical thresholding device being positioned along an optical pathdefined by the propagation direction of the optical input beam and alsoreceiving the control beam, wherein if the intensity of the opticalinput beam is large enough to exceed the threshold level of athresholding device, the thresholding device saturates and turnstransparent so that the control beam passes through the thresholdingdevice as an optical indicator beam and the optical input beam passesthrough the thresholding device.
 9. The optical comparator according toclaim 8 wherein said optical thresholding device comprises a saturableabsorber.
 10. The optical comparator according to claim 8 and furtherincluding means for decreasing the predetermined threshold level. 11.The optical comparator according to claim 10 wherein said means fordecreasing comprises a laser for emitting a threshold control beam intosaid optical thresholding device.
 12. The optical comparator accordingto claim 8 wherein the control beam and the input beam impinge saidoptical thresholding device at an orientation relative to each other sothat they do not interfere with each other in the thresholding device.13. The optical comparator according to claim 8 wherein the input beamand the control beam have substantially the same wavelength.
 14. Theoptical comparator according to claim 8 and further including aphotodetector that is responsive to the optical indicator signal, saidphotodetector generating an electrical signal indicative of theindicator signal.
 15. The optical comparator according to claim 14 andfurther including means for decreasing the predetermined thresholdlevel.
 16. The optical comparator according to claim 15 wherein saidmeans for decreasing comprises a laser for emitting a threshold controlbeam into said optical thresholding device.
 17. An optical comparatorfor comparing an optical input beam with a threshold level, comprising:a semiconductor saturable absorber; and an electrical measuring system,said semiconductor saturable absorber having a predetermined thresholdlevel, being positioned along an optical path defined by the propagationdirection of the optical input beam, and generating charge carriers inresponse to the input beam, wherein if the intensity of the opticalinput beam is large enough to exceed the threshold level of asemiconductor saturable absorber, the saturable absorber saturates andturns transparent allowing the optical input beam to pass through theabsorber, and said electrical measuring system provides an electricalmeasurement of the charge carriers generated by the semiconductorsaturable absorber as an indicator signal of the input beams intensity.18. The device according to claim 13 wherein said semiconductorsaturable absorber is configured as a reversed bias p-i-n structure. 19.A method of comparing an optical input beam to a threshold, comprising:positioning an optical thresholding device having a predeterminedthreshold level along an optical path defined by the propagationdirection of the optical input beam; saturating said opticalthresholding device if the intensity of the input beam is above thethreshold level of the thresholding device, wherein if the intensity ofthe input beam is above the threshold level of the thresholding device,the thresholding device saturates and turns transparent allowing theinput beam to pass therethrough with some attenuation; and providing anindicator signal when the thresholding device has been saturated by theinput beam.
 20. A method of comparing an optical beam to a threshold asrecited in claim 19 and further including selectively lowering thethreshold level of said optical thresholding device.
 21. An opticalsignal attenuator for use with an optical input beam and a control beamcomprising: means for generating a control beam; and an opticalthresholding device having inputs for receiving the optical input beamand the control beam and a transmission characteristic that varies withthe combined intensity of the optical input beam and the control beam,wherein the intensity of the optical input beam is attenuated as itpasses through said optical thresholding device.
 22. The opticalattenuator according to claim 21 wherein the combined intensity of theoptical input signal and the control beam is less than that necessary tosaturate said optical thresholding device.
 23. The optical attenuatoraccording to claim 21 wherein the intensity of the optical input signalis substantially less than the intensity of the control beam.
 24. Theoptical attenuator according to claim 23 wherein the transmissioncharacteristic varies generally monotonically over a range ofintensities of the control beam.
 25. The optical attenuator according toclaim 21 wherein the optical threshold device is a saturable absorber.